Although the block was designed as a security measure to stop Java running on browsers, thus shielding users from potential hackers who can take over their computers using vulnerabilities found on the software, it was also blocking non-browser applications on Friday.

Ivan Glaser, regional chief information officer Asia Pacific, for communications group Havas Worldwide, said some 18 advertising agencies in the region woke up Friday morning to find themselves cut off from their backbone application.

Starflow was developed in-house to manage all aspects of Havas agencies' workflow, from job tickets, to production estimates, timesheets, orders and document management. It does not require use of a browser, but is built on Java, hence agency personnel being unable to use it after Apple effected the block overnight on Thursday. Havas Worldwide is one of the largest global communications groups in the world with more than 300 offices in 75 countries.

"We started getting phone calls from our Melbourne office early this morning, saying they couldn't log on. I originally thought it was a localised issue, but half an hour later people in Sydney couldn't log on."

Glaser then found out from another office that a systems analyst had been up all night developing a temporary fix for another software application, Pegasus, which is also widely used by ad and media agencies worldwide.

"He did some investigation and found out it was a Mac Java block that affected all Mac users," Glaser said, adding that ad agencies rely heavily on Mac computers for their daily work. Across the region Macs account for a fifth of all Havas computers, but some offices use them exclusively.

"Luckily, he found a solution for us which gets around the Apple block and fortunately, because of time difference, we've been able to send it around to all other offices. But not every office has servers they can push the fix to, so it means they have to go around to every single Mac to implement."

A spokesperson for Pegasus in Sydney confirmed the block had affected the application, but declined to comment until more information was available from Apple in due course.

An Apple spokesman in Australia said the company had no comment.

Glaser said he was "astounded" that Apple would effect such block without notice.

"I don't know how many people run mission-critical Java applications, but it's a sign of Apple's omnipresence. Imagine if Microsoft pulled the switch on Exchange … It's OK if Apple blocked the browser, but these applications don't use a web browser."

He said non-Mac users could not communicate with Mac users via the applications.

"It's just irritating. When things stop working, users immediately blame IT, so we are at pains to notify staff of the reason. It's an Apple thing," Glaser said.